Many examples occur, both among animals and among plants, in which families widely removed from one another as to their fundamental structure, nevertheless present a singular, and sometimes extremely close, resemblance in their external characters. Thus the composite Hydroid Zoophytes and the Polyzoa are singularly like one another - so much so, that they have often been classed together; whereas, in reality, they belong to different sub-kingdoms. Many other cases of this resemblance of different animals might be adduced, and in many cases these "representative forms" appear to be able to fill each other's places in the general economy of nature. This is so far true, at any rate, that "homomorphous" forms are generally found in different parts of the earth's surface. Thus, the place of the Cacti of South America is taken by the Euphorbiae of Africa; or, to take a zoological illustration, many of the different orders of Mammalia are represented in the single order Marsupialia in Australia, in which country this order has almost alone to discharge the functions elsewhere performed by several orders. Many homomorphous forms, however, live peacefully side by side, and it is difficult to say whether in this case the resemblance between them is for the advantage or for the disadvantage of either. In other cases we find certain animals putting on the external characters of certain other animals, to which they may be closely related, or from which they may be widely separated in zoological position. Such cases are said to be examples of "mimicry," and such animals are said to be "mimetic." Excellent examples of this may be found amongst certain Butterflies, or in the close resemblance of the clear-winged Moths to Bees and Hornets. In all these cases it appears that the mimetic species is protected from some enemy by its outward similarity to the form which it mimics. Finally, there are numerous cases in which animals resemble certain natural objects, and thus greatly diminish their chances of being detected by their natural foes. Excellent instances of this "protective resemblance" are afforded by the insects known as Walking-leaves (PhyIlium, fig. 2) and Walking-sticks (Phasmidae), which respectively present the most singular resemblance to leaves and dried twigs. The student, however, must carefully guard himself against supposing that the term "mimicry" implies any conscious action on the part of the mimetic species; there being no evidence to support such a view.